We have been studying E. coli DNA topoisomerase I and III for a number of years and have proposed a catalytic mechanism. We are studying the interactions of topoisomerase I with DNA and also the structure of mutant proteins that could shed light on the details of the reaction. We solved the structure of two repeats of chicken brain ?-spectrin in 1997 using data collected at SSRL. The structure revealed the relative arrangement of the two subunits and the nature of the linker region. The original crystals diffract to only 4.0 E resolution and many questions could not be answered from this structure. Nevertheless the structure allowed us to redesign our construct and obtain better crystals. We have now three new crystal forms that diffract to high resolution. The third project is concerned with the structure of domain I of the RNA component of RNase P. This domain is 154 bases long and the crystals diffract to 3.8 E at a synchrotron source. It is one of the largest RNA molecules crystallized and should allow us to answer important questions on the catalytic mechanism of RNase P and also on RNA structure in general.